Having an artifact (for reference) or no, that is a
huge difference. We have a "natural" artifact for the
second - it is a cesium atom. Then we don't need one
for the meter. (The whole planet Earth once was.) Now,
we can avoid artifact for the kilogram also. Just take
a (cesium) atom an tell, how many of them are needed
for a kilogram, or how fast 1 must travel.

But there is another, even better way. Starting from
the Plank's length. A certain number of Planck's
lengths is a meter. A kilogram is a mass of a black
hole with a certain number of Planck's lengths in
diameter. A second is a certain fraction of time
needed, for the unit black hole to evaporate.

Good enough for the posthumanity? I doubt, but having
something good, will be just essential.